Today I went to a panel about a game called Faceball 2000, one of the first FPS, and it's impressive Game Boy port. Although Nintendo only ever released hardware to support 4 player games, Faceball 2000 is programmed to support 16 players.

Jeremy Parish showed up with 16 copies of Faceball 2000, Alexander Bahr showed up with an extremely custom 16 player adaptor. Anyone with a Game Boy or Game Boy derivative was welcome to participate. Although they only managed to scrounge up 9 Game Boys, and could only get a 7 player game going, it's still mighty impressive they got past the officially supported four players.